It's one of the most recognisable guitar riffs in music. But there have been some rather unsavoury things said about Deep Purple's heavy rock anthem, Smoke on the Water, over the years. And then there's the pub covers bands who give the song a bad name as they wade into the opening bars - "da da da, da da da da" - after a long, boozy night.

Yet for Deep Purple singer and co-songwriter Ian Gillan, it's still a special song - and, 40 years on from when it was written, you have to love the way he describes it.

"I think that song is like having a motorbike or a wild horse waiting for you outside," he says. "You just get on it and go for a ride. You feel the wind blowing through your hair, you just climb on board."

And who would have thought it, but the late opera star Pavarotti was also a Deep Purple fan, and he had a few observations about the song for Gillan, who had worked with the great tenor on a number of projects before he died in 2007.

"He told me that he'd seen us play Smoke On the Water six times and each time it was different. He said, 'if I did that with Nessun Dorma they would crucify me'," laughs Gillan. "But when we're touring, every night is a different environment and a different audience, so sometimes [the song] is driving, sometimes it's laid-back ..."

It will be one of the highlights of the band's set when they play Vector Arena on February 24, along with American prog-meets-soft rock survivors Journey.

It sure is a big year for pioneering British - for want of a better term - heavy metal, what with Deep Purple, Black Sabbath, and Robert Plant (though his show is likely to be more bluegrass and worldy than grunty Zeppelin material), playing in New Zealand during the first half of the year.

"Holy moly, that's great ... we're looking forward to it," says Gillan on the phone from Berlin, where the band have just finished the European leg of their latest tour.

The current Deep Purple line-up resembles the one that toured here in 1973 in support of their classic Machine Head album from the previous year, which had Smoke on the Water on it. There's Gillan, bass player Roger Glover and original drummer Ian Paice, but keyboardist Jon Lord, who died last year ("we always looked upon him as Deep Purple's founding father. He was someone you'd always want to go out for dinner with, Jon") and the band's most famous guitarist, Ritchie Blackmore (who left the band in 1994), have been replaced by keyboardist Don Airey and guitarist Steve Morse.

The recording of Machine Head is the stuff of rock 'n' roll legend. "It was fairly well organised, for Deep Purple at least. In other words, we all turned up at the same time and in the same place," remembers Gillan with a laugh.

That place was Montreux, Switzerland, in December 1971. However, the album was meant to be recorded at a casino in the city, but a fire during a Frank Zappa gig burned the place down. Not only did the inferno inspire Smoke on the Water, but on a more practical level, Deep Purple were forced to use a more makeshift recording venue.

"We ended up at the Grand Hotel. The gear was set up in hallways and bedrooms, so the residents hated us, and we had the police banging on the door many a time, and we had to keep them out while we finished the track. It was quite difficult."

And at the end of the recording sessions, they were short by one song to fill an album so, remembers Gillan, "We used a soundcheck track and quickly wrote a biographical lyric about the making of Machine Head, which turned out to be Smoke on the Water. It was just a throwaway track really. But it rounded off the album quite nicely."

That song is what they are best-known for. It was back in 1969, when Gillan and Glover joined, that Deep Purple started taking a more accessible and commercial bent - even though Gillan insists they never had mainstream ambitions.

"We brought an element to the band that was more than just the noise we make," he says with a laugh.

It's that combination of noise, and the diverse influences that they worked into their music, ranging from orchestral, jazz and big band through to rock 'n' roll, soul and blues, that helped pave the way for heavy metal.

"It's quite textured, and it is hard rock, but there is a lot more to it than that."

And the remarkable thing, even for Gillan, is that they are still in demand today.

"It's hard to figure out. We haven't done anything different, apart from keeping the thing alive, and that there's still a lot of energy in what we do. But we just love the music we're playing.

"You're never going to be believe how much we enjoy it. We still turn around and grin at each other [on stage] and ours is the art of improvisation, so every song changes, night by night, town by town."